A block and tacky: is an apparatus comprising a set of pulleys in combination with a single, continuous, linear-tension member for amplifying the tension force in the linear-tension member. A linear-tension member is the tackle in the block-and-tackle combination and is commonly referred to as a line, rope or cord. In one common configuration a first block houses one pulley on a single axle while a second block houses two pulleys on a single axle. The first block is removably engaged with a load. A cord is fixedly engaged with the first block; runs around a first pulley in the second block, then around the single pulley in the first block; following around the second pulley in the second block and then on to the pulling force. In this manner the pulling force is amplified.
An ideal block and tackle provides a mechanical advantage that may be described by the following equation:
                    F        B                    F        A              =    n    ,
Where FA is the tensile force applied to the cord, FB is the load and n is the number of rope sections. In other words, the block and tackle reduces the pulling force by a factor n. A block and tackle with an n factor of 6 will require six units of cord to be pulled to move the load a distance equal to one unit of cord.
One skilled in the art understands that there are efficiency losses in any system and that an ideal system expresses the overall function of a mechanical apparatus. One skilled in the art also understands that there are many variations to a block-and-tackle system with varying numbers of pulleys per block. Variations in the number of pulleys vary the n factor. In a common configuration, the block with the free end of the tackle is mounted to a structure and is referred to as the stationary block, while the second block is attached to the object to be lifted and is referred to as the moving block. In this configuration the pulling force is in the opposite direction of the movement of the object being lifted. One skilled in the art understands that switching the location of the blocks results in a pulling force that is in the same direction as the movement of the object being lifted.